Device and process for producing a block of crystalline material

ABSTRACT

A temperature gradient is established in a crystallization crucible by means of a heat source and a cooling system. The cooling system comprises a heat exchanger and an adjustable additional heat source. The cooling system is preferably formed by an induction coil cooled by a coolant liquid circulating in the induction coil and by an electrically conductive induction susceptor positioned between the crucible and induction coil. The fabrication process comprises heating the crucible via the top and controlling heat extraction from the crucible downwards by means of the heat exchanger and by means of regulation of the adjustable additional heat source.

This is a Continuation of application Ser. No. 12/087,308 filed Jul. 1,2008, which in turn is a U.S. National Phase of PCT/FR2006/002661 filedDec. 6, 2006, which claims the benefit of French Patent Application No.06 00049 filed Jan. 4, 2006. The disclosures of the prior applicationsare hereby incorporated by reference herein in their entirety.

BACKGROUND OF THE INVENTION

The invention relates to an equipment for the fabrication of a block ofcrystalline material by directional crystallization comprising heatingmeans and cooling means arranged such as to establish a temperaturegradient in a crystallization crucible, the cooling means comprising aheat exchanger and an adjustable additional heat source.

STATE OF THE ART

The document WO2004/094704 describes an equipment for fabrication of ablock of crystalline material by directional crystallization. Thismaterial is typically silicon. As represented in FIG. 1, the devicecomprises a crystallization crucible 1 the bottom 7 of which enablesheat to be extracted. The bottom 7 of the crucible 1 has greater thermaltransfer properties than those of the side walls 8. To generate atemperature gradient, a heating element 3 and a heat exchanger 4 arerespectively arranged above and below the crucible 1.

The temperature gradient for the solidification of silicon requires anefficient heat extraction. The anisotropic properties of the crucible 1enable substantially flat and parallel isothermal planes to be obtained.The solidification front is consequently substantially flat in adirection parallel to the bottom 7 of the crucible 1.

When crystallization of the silicon takes place, the thickness of thesolid phase 5 increases so that the solidification front progressesupwards moving away from the bottom 7 of the crucible 1, as representedby the arrow 20 in FIG. 1. The melting temperature of silicon being1410° C., the 1410° C. isothermal plane then moves away from the bottom7 of the crucible 1, resulting from the temperature decrease at thebottom 7 of the crucible 1 during the crystallization process.

The increase of the quantity of solidified material at the bottom 7 ofthe crucible 1 is accompanied by an increase of the thermal resistance.To keep the temperature gradient at the solid/liquid interface constant,the thermal power removed by the heat exchanger 4 needs however have toremain substantially constant throughout the solidification sequence.Therefore an adjustment then has to be provided.

The heat exchanger 4 thus comprises for example a coolant fluid circuitand, depending on the applications, the fluid can be synthetic oil or afluid operating at high temperature, for example a gas under pressuresuch as helium, which can be very costly when a helium liquefier isrequired. It is possible to make the temperature of the coolant fluidvary in a controlled manner to ensure that the power removed remainsconstant throughout the solidification sequence.

Document WO2004/094704 for example describes a graphite felt 9 arrangedbetween the bottom 7 of the crucible 1 and the cooling means 4, asillustrated in FIG. 2. The graphite felt 9 is compressed duringsolidification of the silicon. The thickness of the graphite felt 9thereby decreases and its thermal conductivity increases. Heat transferby conductivity of the graphite felt 9 can then be controlled during thesolidification process. The graphite felt 9 can also be progressivelyremoved to control the cooling. The temperature gradient in the crucible1 is typically controlled and kept at a value comprised between 8° C./cmand 30° C./cm.

Water cooling systems also exist. However it is difficult to control thetemperature over a wide temperature range unless the latent heat fromwater vaporization is used, which is complicated to implement.

Document U.S. Pat. No. 6,299,682 describes an apparatus for producing asilicon ingot for photovoltaic applications. The apparatus comprises aheating system arranged above a crucible and cooling means arrangedbelow the crucible. A heat source is also arranged under the crucible tocontrol the heat removed at the bottom of crucible.

OBJECT OF THE INVENTION

The object of the invention is to remedy these drawbacks and inparticular to achieve an efficient temperature adjustment while at thesame time reducing the implementation costs of the device.

According to the invention, this object is achieved by the appendedclaims and more particularly by the fact that the cooling means areprovided by an induction coil cooled by a cooling liquid flowing in theinduction coil and by an electrically conductive induction susceptorpositioned between the crucible and the induction coil.

It is a further object of the invention to provide a process forfabricating a block of crystalline material by directionalcrystallization using an equipment according to the invention, theprocess comprising:

-   -   heating the crucible via the top and cooling the crucible via        the bottom to establish the temperature gradient in the        crystallization crucible, and    -   controlling the heat extraction from the crucible via the bottom        by means of the heat exchanger and by means of regulating of the        adjustable additional heat source,        the cooling means being provided by an induction coil cooled by        a cooling liquid circulating in the induction coil and by an        electrically conducting induction susceptor arranged between the        crucible and the induction coil,    -   the process simultaneously comprising heating by means of the        induction coil and cooling by means of the cooling liquid        flowing in the induction coil, and    -   the process comprising progressive reduction of the heating by        means of reduction of the electric power supply of the induction        coil, while a solidification front progresses in the crucible        moving away from the susceptor.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features will become more clearly apparent from thefollowing description of particular embodiments of the invention givenas non-restrictive examples only and represented in the accompanyingdrawings, in which:

FIG. 1 shows an equipment for the fabrication of a block of crystallinematerial by directional crystallization according to the prior art.

FIGS. 2 and 3 show two particular embodiments of the equipment accordingto the invention, in cross-section.

FIG. 4 illustrates a particular embodiment of a susceptor of a deviceaccording to the invention, in top view.

DESCRIPTION OF PARTICULAR EMBODIMENTS

The fabrication equipment represented in FIG. 2 comprises a heat source3 and a cooling system 22 arranged such as to establish a temperaturegradient in the crystallization crucible 1. The cooling system 22comprises a heat exchanger 17 and an adjustable additional heat source18, for example resistive heating elements, infrared heating or anyother suitable adjustable heating. The heat exchanger 17 preferablycomprises a water cooling circuit represented schematically by arrow 21.The heat exchanger 17 can in particular be extended over the whole widthof crucible 1.

The heat extraction from the crucible can thus be controlled, while atthe same time benefiting from the advantages of a constant power heatexchanger, in particular its simple implementation.

The equipment can also comprise a removable felt 19 positioned above theheat exchanger 17 and under the adjustable additional heat source 18,thereby forming a screen preventing thermal radiation between the heatsource 18 and the heat exchanger 17, thus enabling the heat extractionfrom the crucible 1 to be further controlled. By placing the removablefelt 19 between the heat source 18 and the heat exchanger 17, the heatextraction is reduced, whereas removing the felt 19 allows for directevacuation of the radiation emitted by the source 18 in the direction ofthe heat exchanger 17.

In the preferred embodiment represented in FIG. 3, the cooling system 22comprises an induction coil 10 cooled by a coolant liquid circulating inthe induction coil 10 and an electrically and thermally conductinginduction susceptor 11 positioned between the crucible 1 and theinduction coil 10.

The combination of the induction coil 10 and induction susceptor 11 thusprovides an adjustable additional heat source 18. Adjustment isperformed by means of the electric power supply 24 (see FIG. 3) for theinduction coil 10. At the same time, the heat exchanger 17 is formed bythe unit comprising the induction coil 10 and its cooling circuitconsisting of a cooling liquid flowing in the induction coil 10,illustrated by arrow 21 in FIG. 3.

A heat source 3, for example formed by resistive heating elements, by anadditional induction coil and its additional susceptor or by any othersuitable heating means such as infrared heating for example, ispositioned above the crucible 1. The induction coil 10 is arranged underthe crucible. The induction susceptor 11 is situated between thecrucible 1 and the induction coil 10. The crucible 1 preferablycomprises a transparent bottom 7 made from impurity-free amorphoussilica enabling infrared radiation to be transmitted. When a removablefelt 19 is used as described above, the removable felt 19 can bepositioned between the induction coil 10 and susceptor 11.

Induction coils according to the prior art are typically cooled by acooling liquid circulating in the induction coil. Heating inductioncoils are however conventionally not in thermal contact with the objectto be heated, and may even be thermally insulated from this object.Cooling therefore only acts on the coil itself and cannot act on theobject to be heated.

The susceptor 11 is preferably made from thermally conducting materialwhich absorbs infrared radiation, for example graphite or siliconcarbide which are conductors and good black bodies. The heat emitted bythe bottom 7 of the crucible 1 is thus absorbed by the face 12 of thesusceptor arranged facing the crucible 1, transported through thesusceptor 11 and re-emitted via the face 13 of the susceptor 11 arrangedfacing the induction coil 10. The induction coil 10 enables the heat tobe evacuated. The susceptor 11 can form a support for the crucible 1 andenables a good thermal exchange to be obtained between the susceptor 11and crucible 1. The induction coil 10 is preferably placed in anelectrically insulated zone, using for example silica plates 23 toprevent short-circuits and formation of sparks to the neighboringgraphite 14. As represented in FIG. 3, silica plates 23 are arrangedaround the induction coil 10.

The susceptor 11 can simply be formed by a flat plate. In a particularembodiment, the susceptor 11 comprises zones 15 of lower electricsurface conductivity enabling induction heating to be locally reducedand also locally provoking heat extraction from the crucible 1 to theinduction coil 10. The zones of crucible 1 situated opposite to zones 15of lower electric surface conductivity are consequently less heated andthereby form nucleation centres for crystallization. In addition, thezones of crucible 1 opposite to zones 15 of lower electric surfaceconductivity are better cooled, since these zones 15 of lower electricsurface conductivity also locally provoke heat removal from the crucible1 towards the induction coil 10.

Zones 15 of lower electric surface conductivity are for example formedby holes arranged in the susceptor 11, as represented in FIG. 4. Zones15 of lower electric surface conductivity can also be formed by amaterial having a lower electric conductivity than the material ofsusceptor 11 or by a zone having a smaller thickness than the susceptor.The lateral dimension of zones 15 of lower electric surface conductivityis preferably equal to or greater than the thickness of the susceptor.The distance between zones 15 of lower electric surface conductivity isfor example 10 cm for a susceptor of a few tens of decimeters in size.

A process for the fabrication of a block of crystalline material bydirectional crystallization using an equipment according to theinvention comprises heating of the crucible 1 via the top and cooling ofthe crucible 1 via the bottom to establish the temperature gradient inthe crystallization crucible 1. The process further comprises heatextraction from the crucible 1 downwards by means of the heat exchanger17 and by means of a regulated additional and adjustable heat source 18.The heat exchanger 17 can in particular operate in constant regime,which simplifies the implementation of the heat exchanger 17.

The process for the fabrication of a block of crystalline material bydirectional crystallization using the equipment according to theinvention can in particular comprise heating by means of the inductioncoil 10 and its susceptor on the one hand, and at the same time coolingby means of the cooling liquid flowing inside the induction coil 10 onthe other hand. While a solidification front 16 progresses in thecrucible 1 moving away from the susceptor 11 in the upward direction asrepresented in FIG. 3 by arrow 20, heating is progressively reduced byreducing the electric power supply to the induction coil 10, whereas thecooling liquid can circulate flow in constant manner inside inductioncoil 10.

Before crystallization, the process can also comprise a melting step ofthe material to be crystallized using heat source 3 and adjustableadditional heat source 18 comprising for example induction coil 10 andits susceptor 11. This in particular enables the material to becrystallized to be completely melted.

The invention is not limited to the embodiments represented. Inparticular, the gradient is not necessarily established from top tobottom and may be oriented along any axis, for example horizontal orinclined. In the latter case, the heating and cooling means arerespectively arranged on each side of the gradient zone.

1. A device for the fabrication of a block of crystalline material bydirectional crystallization, the device comprising: heating means andcooling means arranged such as to establish a temperature gradient in acrystallization crucible, wherein the cooling means are arranged underthe bottom of the crystallization crucible so as to cover the bottom ofthe crystallization crucible and comprise: a heat exchanger and anadjustable additional heat source, an induction coil cooled by a coolantliquid circulating in the induction coil and an electrically conductinginduction susceptor positioned between the crystallization crucible andconduction coil.
 2. The device according to claim 1, wherein thesusceptor is made from a thermally conducting material that absorbsinfrared radiation.
 3. The device according to claim 2, wherein thesusceptor is made from one of graphite and silicon carbide.
 4. Thedevice according to claim 1, wherein the susceptor forms a support forthe crystallization crucible.
 5. The device according to claim 1,further comprising silica plates arranged around the induction coil. 6.The device according to claim 1, wherein the crystallization cruciblecomprises a transparent bottom made from impurity-free amorphous silica.7. The device according to claim 1, further comprising a removable feltpositioned above the heat exchanger.